This invention relates to wired security equipment of the type commonly used in buildings to provide audible or similar alarm when an intruder enters the premises.
Existing wired security equipment generally comprises a plurality of electrical contacts each mounted on a respective door or window of the premises so as to be operated when that door or window is opened. The equipment may also incorporate floor pressure mats which open or close contacts when stepped upon. The door and window contacts may be of the normally closed or normally open types, and a known equipment utilizing normally-closed types is shown by way of example in FIG. 1 of the accompanying drawings.
In FIG. 1, a plurality of pairs of normally closed contacts S1 to S4 are connected in series to form a closed loop, known as the "detection circuit". A second closed loop, known as the "monitoring circuit" accompanies the detection circuit in close physical proximity therewith. Because of this close physical proximity, any damage, whether accidental or otherwise, to the detection circuit is likely to affect the monitoring circuit as well, thus enabling such damage to be detected.
Both detection and monitoring circuits terminate in a control circuit 1 which is able to detect, by a cessation in current flow, when any one or more of the contacts S1 to S4 are opened. Similarly any damage to the monitoring circuit resulting in a break in the circuit may also be detected by the control circuit 1. Both these conditions will result in some form of alarm, usually audible, being operated.
The equipment shown in FIG. 1 can also be used to detect the closure of any normally open contacts S5 connected as shown between the detection and monitoring circuits. An example of such a device is a floor pressure mat, referred to above, which latter would usually possess normally open contacts.
One problem with the equipment shown in FIG. 1 is that, once one normally closed contact has been opened, the states of the remaining contacts cease to be monitored since there is no current flow. Furthermore, the monitoring circuit, if fitted, is an expensive and cumbersome adjunct which, whilst being extremely effective in high-security installations, can only increase the obstrusiveness of the wiring.